1 Taken from "Heating & Ventilating," by Harding and Willard. Problems 1. How much heat is required to change 1 lb. of water at 32°F. into steam at a pressure of 10 lb. gage? To raise the temperature of the water from 32°F. to its boiling temperature (at the pressure of 10 lb. gage) requires 208 B.t.u. TABLE 3.-FLOW IN GALLONS PER MINUTE DELIVERED BY ORDINARY PLUMBING FIXTURES To check this estimate, we can utilize the formula given This is a fairly close check, and to be on the safe side we will use this figure as representing the maximum probable demand per minute. Now the heater need not have a capacity large enough to take care of this demand. In most cases a storage tank will be used whose capacity will be about equal to the full demand. In other words, a heater whose capacity is about equal to 1/4 G = 2.2 gal. per minute, would undoubtedly meet the requirements of this problem. The heating surface (coils in heater) for a heater of this capacity (2.2 gal. per minute), can be computed from A = Q where A = square feet of heating surface. K to tw = B.t.u. transmitted to water per hour per 1° difference in temperature between water and gas, = about 2. = = = = = average temperature of gas. 1⁄2 X sum of gas temperature entering and gas temperature leaving, about 1,500°F. average temperature of water, 1⁄2 X sum of water temperature entering and water temperature leaving, This value is only approximate, but affords a rough idea of what heating surface is required in the heater. In most cases, the hourly capacity of the heater is much less than the capacity of the tank, so that the hot water demand on the latter must not be constant, but must permit of periods when the heater can "catch up" by working on the tank alone. In case the hot water demand is practically constant, a much larger heater, suitable for continuous service, must be installed, although the constant rate of supply may be no greater than the intermittent rate provided for above. 1 Carefully note that Q is figured on the assumption that only about onefourth the total gallons of water required per hour should equal the heater capacity. Hence, Q = 4 × G × 8 × (140 – 60) = 72.594 B.t.u. in above problem. A CHIMNEYS1 In order to cause the necessary amount of air to flow through the fuel bed and force the products of combustion through the gas passages of a boiler, a difference in pressure between the ashpit and the breeching is required. This difference in pressure is known as draft and is due to the difference in weight of the hot gases in the chimney and the cold air without. In Fig. 4 are shown two chimneys of equal height and cross-section area, side by side, and connected at the bottom by passage C. If we locate a steam radiator at the base of stack B and let steam into the radiator, there will immediately result an upward movement of air in B, and at the same time a downward flow of cold air through A; that is, as the air in B is heated, it expands, and its weight per cubic foot becomes less than that of the air in A; hence, the column of cold air in A being heavier than the hot air in B, the latter is forced up and out of B by the colder air of A. In effect, this is precisely what occurs in the case of an actual chimney, although in that case there is not a duplicate chimney. The intensity of this draft, or the difference in pressure between the hot gases in the chimney and the outside air, can be determined by means of the arrangement shown in Fig. 5. A bent piece of 1-in. pipe is inserted into flue at point f. On the other end is attached a piece of rubber tubing, d, and this in 1 See "Sheet Metal" section, page 403. Flue Between the turn is connected to the end of a glass U-tube, b. legs of the U-tube is a scale, c, marked off in inches and tenths of an inch. The U-tube and scale can be mounted on a piece of board,a, and the whole fastened in any convenient place in close proximity to the flue. If water is poured into the U-tube, it will be observed that the level of the water in the right-hand leg will be higher than that in the left-hand leg. This is due to the difference in pressure between the flowing gases in the flue and the outside air. The difference, hw, between the two levels increases with the intensity of the draft. The draft at the rear of the boiler where connection is made to the flue, may be 0.5 in. of water, while in the furnace, directly over the fire, it may not exceed 0.1 to 0.15 in. of water, the differ 2nd Floor Ist. Floor Basement FIG. 6. ence being the draft required to overcome the resistance offered to the flow of the gases through the various passages of the boiler. In order to secure sufficient draft to maintain satisfactory burning of the fuel, it is necessary that the chimney shall have the proper height (H in Fig. 6). H can be calculated from formula below. effective height of chimney measured in feet from furnace grate to top of chimney (see Fig. 6). velocity of flow of hot gases up chimney in feet per second (see Table 5). temperature of hot gases in chimney in degrees Fahrenheit. temperature of outside air in degrees Fahrenheit. |